1. Field of the Invention
The present invention relates to a malfunction avoiding guard apparatus for avoiding the malfunctions of a communication apparatus due to an error in the received data in the specific location of a frame in an optical communication system, in particular, a SONET/SDH optical communication apparatus.
2. Description of the Related Art
In a SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) optical communication apparatus, for example, the K1 and K2 bytes of the line overhead unit of a SONET frame are assigned to automatic protection switching (APS) signals between line termination devices, and on the receiving side of the frame the communication apparatus is controlled according to the contents extracted from these bytes.
Since when the contents of the K1 and K2 bytes are changed due to an optical degradation in the communication system, there is a possibility that the communication apparatus malfunctions, and conventionally, in order to avoid this malfunction, data are considered to be correct, and are extracted, only if the data of the K1 and K2 bytes are the same in three consecutive frames.
In this conventional way, even when there is an error in data due to an optical degradation, the data are considered to be correct and are extracted, as long as the same data are received in three consecutive frames. Thus, conventionally there is a problem that this causes malfunctions of the communication apparatus.
It is an object of the present invention to prevent erroneous data caused by an optical degradation from being extracted, which is a cause of the malfunctions of a communication apparatus, by changing a protection method depending on whether or not an error predetermined for a receiving frame is detected and to reinforce the guard for the error to be stronger than ever when the error is detected.
The guard apparatus of the present invention for avoiding the malfunctions of a communication apparatus due to an error in the received data on the receiving side of an optical communication system comprises a tri-stage protection unit and a selector unit.
If the data in the predetermined location of the received frame and the data in the above-mentioned predetermined location of the received frames received one and two frames before the above-mentioned frame are all the same, the tri-stage protection unit outputs the data, while if not, or if an INH signal is inputted, the tri-stage protection unit outputs the data outputted for a received frame received one frame before the above-mentioned frame.
If an error detection signal predetermined for the above-mentioned received frame is inputted, the selector unit outputs the above-mentioned INH signal to the above-mentioned tri-stage protection unit and selects the output of the tri-stage protection unit, while if not, the selector unit selects and outputs the data in the predetermined location of the above-mentioned received frame.
In the present invention, the guard for avoiding malfunctions can also be reinforced by further providing a second tri-stage protection unit. If the output data outputted by the selector unit and two output data outputted by the above-mentioned selector unit one and two data before the above-mentioned output data are all the same, the second tri-stage protection unit outputs the above-mentioned data. While if not, the tri-stage protection unit outputs the data outputted by the selecting unit one data before the above-mentioned output data.
Moreover, in the present invention, for example, an error condition setting unit for providing the selector unit with a pseudo-error detection signal according to an instruction from a computer and an error detection signal nullification unit for nullifying an error detection signal inputted in the selector unit and enabling the selector unit to select data in a receiving frame can also be provided.
According to the present invention, a protection method is changed depending on whether or not an error predetermined for a receiving frame is detected. Thus, the extraction of erroneous data due to an optical degradation being a cause of the malfunctions of a communication apparatus can be avoided.